1. Field of the Invention
The present invention relates to a method of manufacturing a thin film transistor (TFT) comprising a polycrystalline silicon (Si) layer, a TFT prepared using the method and a flat panel display device comprising the TFT, and more particularly, to a method for manufacturing a TFT that provides TFTs with a more uniform threshold voltage by controlling the uniformity of a surface of amorphous silicon film before crystallizing the amorphous silicon film.
2. Discussion of the Background
Generally, amorphous silicon may be crystallized at a low temperature to form a polycrystalline Si layer for a TFT's active layer. There are a variety of methods for crystallizing the amorphous silicon film to obtain the polycrystalline Si layer.
A common crystallizing method uses a laser since heat from the laser may not damage a glass, insulating substrate, and this method may form polycrystalline Si that has better characteristics than polycrystalline Si formed using a solid phase crystallizing method. However, when using a laser crystallizing method, a rough oxide layer, which is formed naturally on amorphous silicon film, may deteriorate TFT threshold voltage uniformity.
With the laser crystallizing method, protrusions may be formed on polycrystalline Si layer surfaces where crystallization is delayed due to density differences when the Si changes from a liquid to a solid. These protrusions may be ½ to 2 times as high as the polycrystalline Si layer's thickness.
Consequently, the protrusions may cause defects in a subsequent fabrication process. For example, when sequentially depositing a gate insulating layer and a metal material for a gate electrode on the polycrystalline Si layer, the gate insulating layer and the metal material may be formed with protrusions that are identical to the polycrystalline Si layer's.
Such protrusions in the gate insulating layer may reduce a breakdown voltage and increase a leakage current. Also, the gate insulating layer's protrusions may cause small protrusions in an aluminum gate electrode, thereby deteriorating the TFT's characteristics. Additionally, the protrusions may result in non-uniform etching and exposing, thereby reducing the TFT's reliability.
Japanese Patent Laid-Open publication 1999-354801 discloses a method of forming a polycrystalline Si layer in which, after forming an oxide layer by washing an amorphous silicon film with ozone water, the oxide layer is completely removed by washing with fluoric acid.
However, the polycrystalline Si layer formed using the washed amorphous silicon film according to this method may have a low threshold voltage.
An active matrix organic light emitting display (AMOLED), which is one of many flat panel display devices that uses a TFT, displays images by controlling a current, flowing through an organic electroluminescence (EL) layer, with a gate voltage of a TFT disposed in each pixel. Therefore, it is important that the TFTs have substantially uniform gate voltages and current characteristics. Hence, the TFTs should have substantially uniform threshold voltages. Therefore, a method for providing the amorphous silicon with a substantially uniform surface, in order to improve threshold voltage uniformity, is required.